In the art, radiation exposure of persons moving in a radiation environment is determined by means of a mobile dosimeter carried by the respective person. An absorbed dose measured by the mobile dosimeter in units of Gray (1 Gy=1 J/kg) is translated into the dose equivalent which has been designed to represent the stochastic biological effects of ionizing radiation on human tissue and which is measured in units of Sievert (1 Sv=1 J/kg)). This translation between absorbed dose and dose equivalent is performed on the basis of Linear Energy Transfer (LET) dependent quality factors (Q factors). Accordingly, having accurate knowledge of the deposited energy of particles in the mobile dosimeter is of key importance in determining the dose equivalent, which alone allows assessing the physiological impact of the radiation.
However, radiation fields may significantly vary, particularly in space, with regard to aspects such as their particle content, energy spectrum of particles, intensity of particle flux, and the like. Accordingly, mobile dosimeters are typically adapted for specific use in a given type of radiation environment. If the kind of particles present in the radiation environment is known (e.g. predominantly alpha particles, predominantly protons, predominantly electrons, predominantly neutrons or predominantly gamma rays, or known compositions of any of the aforementioned), and also the typical energy distribution and particle flux is known, the mobile dosimeter can be calibrated to perform optimally in the respective known radiation environment. Rather accurate knowledge of the expected radiation environment is typically the case e.g. in nuclear power plants during normal operation or in laboratories in which medical equipment such as an x-ray apparatus is operated.
On the other hand, due to their specific calibration, mobile dosimeters may yield inaccurate results if the actual radiation environment deviates from the expected radiation environment for which the mobile dosimeter had been calibrated. As a result, a determined dose equivalent may deviate from the actual dose equivalent received by the person carrying the mobile dosimeter. For astronaut health monitoring as well as for any occupational health and safety monitoring, self-evidently, this situation is undesirable and has to be corrected to minimal acceptable limits.
Therefore, an accurate knowledge of the actual characteristics of the radiation environment is of key importance. Most known mobile dosimeters for ionizing radiation however are passive dosimeters and need offline analysis for determining the so-called side variables related to characteristics of the radiation environment. In these mobile dosimeters, a possible deviation between the expected radiation environment and the actual radiation environment can only be determined after exposure and post processing (i.e. on ground as regards human spaceflight).
Active dosimeters known in the art on the other hand do also not provide side variables online, so that there is a possibility that a mismatch between the specific calibration of the dosimeter and an optimal calibration for the actual radiation field may not be immediately detected. Hence, the accuracy of the dosimeter may be reduced, which may be only recognizable after exposure and post processing.
Accordingly, using conventional dosimeters in a radiation environment that is either not known to begin with, or that may change over time, there is a risk that harmful radiation is overlooked and that moreover the dose equivalent received by the relevant person under surveillance is not determined correctly by the mobile dosimeter.
Such a radiation environment in which conventional mobile dosimeters are expected to fail is present e.g. in space stations orbiting earth at a low earth orbit (LEO), and also generally in space. The radiation field in low earth orbit is known to be the most complex natural radiation environment encountered by humans. A further environment in which conventional mobile dosimeters are expected to fail may be present on a site of a nuclear accident, such as the recent Fukushima accident.
The radiation environments mentioned above may also vary with regard to their characteristics with position, and can change with time, which triggers another challenge to conventional mobile dosimeters.